To place heat-shrink sleeves on traveling articles, it is conventional to use a technique whereby the sleeves are cut from a continuous sheath that passes over a sheath-opening shaper, which shaper is held floating by co-operation between outer wheels and backing wheels of parallel axes carried by the shaper, which outer wheels serve to cause the sheath to advance along the shaper (which is generally vertical) up to and beyond cutter means. Other wheels are generally provided downstream from the cutter means to eject the segment of sheath that has been cut off onto the article that travels to a position vertically beneath the shaper.
Thus, in most of the techniques used, there are first outer wheels for advancing the sheath over the shaper, and second outer wheels serving to eject the cut-off sheath segments onto the articles in question. All of the outer wheels are naturally motor-driven, and the way they are motor-driven has given rise to various types of arrangement.
Thus, proposals have been made for the motor drive of the first and second wheels to be completely independent so as to enable the second wheels to turn much faster than the first, thereby causing the cut-off sheath segment to drop vertically more quickly onto the article in question. That approach is illustrated in document EP-A-0 109 105. In another approach, the rotary drive of the first and second wheels is synchronized, as shown in document EP-A-0 000 851.
Nevertheless, it has been found that the above-mentioned techniques impose limits in terms of rates of throughput, since when high rates are reached, it is found that the sheaths are frequently poorly positioned on the articles, particularly when they constitute sleeves of considerable height.
More recently, an important advance has been made by a technique implementing synchronous control over the electric motors concerned by means of a common electronic programmer arranged to determine a continuous speed variation profile so as to control the ejection of each sheath segment, said programmer including at least one control card that co-operates with an adjacent coder mounted at the end of a shaft that is driven in rotation by a central motor and gearbox unit. This is illustrated in document WO-A-99/59871 in the name of the Applicant. According to that technique, the synchronization makes it possible to envisage rates of throughput that are higher than before, and this is possible with sleeves of a diameter that is hardly any greater than the maximum diameter of the articles.
Nevertheless, there is an increasing demand for ever higher rates of throughput, commonly reaching values of 300 to 600 strokes per minute.
It is then preferable to use machines that are further improved, abandoning the system whereby articles advance stepwise, and also abandoning the coder system mounted at the end of a shaft driven in rotation by a central motor and gearbox unit (as described in above-mentioned document WO-A-99/59871), and instead to make use of a virtual shaft common electronic programmer for controlling all of the electric motors, with the instruction for ejecting a cut-off sheath segment being given by a cell when the traveling article goes past it.
In parallel with this search for very high rates of throughput, there is also a trend to use sheaths made of heat-shrink film that is of ever smaller thickness. As an indication, conventional techniques used to use heat-shrink films with thickness of the order of 50 micrometers (μm), whereas nowadays it is desired to use films of heat-shrink plastics material that is of smaller thickness, i.e. possibly as little as 25 μm, and that is also of smaller density.
The two above-mentioned requirements thus considerably complicate organizing sleeve-placing devices, and mention can be made of one type of technical problem that is becoming more and more awkward, and this relates to the process of cutting the sheath while it is held stationary on the sheath-opening shaper.
The cutter means traditionally used make use of at least one blade turning about the shaper level with a deep groove in the shaper, which groove is associated with a ligament uniting the two component portions of the shaper, the or each blade being caused to pivot in alternation between its retracted position and its cutting position by means of a circumferential type cam system in which a wheel associated with the or each blade runs along a cam path defined over 360° by coaxial tracks in a stationary ring. It has been found that such an arrangement becomes impractical at very high rates of throughput, and that the rapid wear of the wheels is very difficult to control. By way of example, one such arrangement is shown in FIG. 2.
In addition, during the cutting process, the cutting edge of the blade engages the wall of the sheath at the level of the above-mentioned groove in the shaper, and exerts considerable thrust in order to pass through the wall of the sheath. It has been found that the effect of this radial force is to push the lips beside the cut back into the above-mentioned groove, thereby giving rise to a cut edge that is not perfectly straight but has irregularities, and also to generate an inherent risk of the sheath suffering indentation deformations that necessarily disturb the normal process of the sheath advancing and of the cut-off segment being ejected. This negative effect of the wall of the sheath being pushed back while it is being cut becomes even more acute when sleeves are used that are made of a heat-shrink film that is of small thickness and low density.
The state of the art relating more specifically to systems for cutting a continuous sheath in machines for placing sleeves on traveling articles is also illustrated in the documents mentioned below.
Document U.S. Pat. No. 5,566,527 illustrates a cutter system having a knife blade mounted on a turntable, and engaging the wall of the sheath for cutting in a purely radial manner.
Document DE-A-297 16 624 shows a cutter system having multiple coplanar blades that are actuated individually by pneumatic means. There also, the wall of the sheath is engaged by each blade by exerting a thrust force so as to pass through the blade.
Finally, WO-A-2008/076718 describes two different cutter systems, one with a turntable configuration and the other with a system of superposed rings, in which the difference in rotation acts on a finger for driving each pivoting blade, with the wall of the sheath being engaged in purely radial manner by each blade, thereby resulting in considerable thrust being exerted on said wall. An arrangement analogous to the above-mentioned system is to be found with the same drawbacks in document EP-A-1 797 984.